Why does the water becomes gray when I move the viewport?
It looks like you have your Normals pointing in the wrong direction. Check your normals and ensure they are pointing ‘out’ of the water.
With the normals pointing into the body of water it will be rendering as if the fluid is outside a cavity of air. What you’re seeing is Total Internal Reflection as the light is refracted at the surface - the wrong way to what you’re expecting.